There has been a growing interest in the application of microfluidic systems to a variety of technical areas. For example, use of microfluidic systems for the acquisition of chemical and biological information presents certain advantages. In particular, when conducted in microfluidic volumes, complicated biochemical reactions and processes may be carried out using very small volumes of fluid. In addition to minimizing sample volume, microfluidic systems improve the response time of reactions and reduce reagent consumption. Furthermore, when conducted in microfluidic volumes, a large number of complicated biochemical reactions and/or processes may be carried out in a small area, such as in a single integrated device. Examples of desirable applications for microfluidic technology include analytical chemistry; chemical and biological synthesis, DNA amplification; and screening of chemical and biological agents for activity, among others.
Of the several different methods that have been developed for producing microfluidic devices, many utilize adhesives in their fabrication. For example, traditional methods for constructing microfluidic devices borrowed techniques borrowed from the silicon fabrication industry. According to these techniques, microfluidic devices have been constructed in a planar fashion and covered with a glass or similar cover materials to enclose fluid channels. Representative devices are described, for example, in some early work by Manz, et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). In these publications, microfluidic devices are constructed by using photolithography to pattern channels on silicon or glass substrates and etching techniques to remove material from the substrate to form the channels. A cover plate is bonded, typically using adhesives or anodic bonding, to the top of such a device to enclose the channels and contain a fluid flow.
More recently-developed methods that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials typically still require bonding of a cover to enclose fluidic channels. Such methods include micromolding of plastics or silicone using silicon as the mold material (see, e.g., Duffy et al., Anal. Chem. (1998) 70: 4974-4984; McCormick et al., Anal. Chem. (1997) 69: 2626-2630); injection-molding; and micromolding using a LIGA technique (see, e.g., Schomburg et al., Journal of Micromechanical Microengineering (1994) 4: 186-191), as developed at the Karolsruhe Nuclear Research Center in Germany and commercialized by MicroParts (Dortmund, Germany). LIGA and hot-embossing techniques have also been demonstrated by Jenoptik (Jena, Germany). Imprinting methods in polymethylmethacrylate (PMMA) have also been described (see, e.g., Martynova et al., Anal. Chem. (1997) 69: 4783-4789). Although mechanical or other attachment means may be employed, adhesives are commonly used to join a cover to a channel-containing planar microfluidic device.
While microfluidic devices fabricated with adhesives may be suitable for certain applications, their extension in other applications would be problematic. For example, adhesives may be susceptible to undesirable interaction with certain chemical solvents, particularly organic solvents. Undesirable interaction between such solvents and adhesives may vary in scope from relatively benign chemical reaction to more serious dissolution or chemical breakdown, leading to observable effects ranging from skewed detection results to structural failure of devices. Aside from these chemical interaction effects, adhesives in microfluidic devices may also interfere with biological reactions, such as, for example, non-specific binding studies.
Another potential limitation of adhesives—particularly pressure-sensitive adhesives—in fluidic devices is that adhesives are characterized by limited bond strength, which in turn limits the range of fluid pressures with which such devices may be operated. Typical pressure-sensitive adhesives are unsuitable for use in fluidic devices intended to handle pressures greater than approximately 100 psi (689.5 kPa). In applications such as liquid chromatography, it would be desirable to pack fluidic devices with stationary phase materials (e.g., using a slurry containing fine particles) at pressures greater of 500 psi (3450 kPa) (and preferably even higher pressures), and to operate such devices at pressures much greater than 100 psi (689.5 kPa).
Although methods have been developed for fabricating microfluidic devices in selected materials without using adhesives, such methods are not without other drawbacks. For example, multiple issued patents disclose the use of tin-enhanced polyimide materials to fabricate microfluidic structures. One such patent is U.S. Pat. No. 5,932,799, issued to Moles (“the Moles reference”). There, multi-layer microfluidic analyzer modules are fabricated without adhesives by directly bonding polyimide material layers enhanced with tin in a concentration between 400-10,000 ppm. Channels are formed in the surfaces of one or more layers by micromachining techniques such as photolithographic patterning followed by etching. Such layers are then physically compressed at a pressure between 24-690 bar (348-10,000 psi), preferably under vacuum at a sub-atmospheric internal pressure, and heated to a temperature between 350-455° C. for a period of about 5 minutes to 3 hours. The resulting structure of tin-containing polyimide material defines internal microfluidic channels along the interface between layers, and may be used to aid in detecting the presence of analytes.
Another reference describing adhesiveless microfluidic laminate structures fabricated from tin-enhanced (400 to 10,000 ppm) polyimide layers is U.S. Pat. No. 6,156,438, issued to Gumm, et al. (“the Gumm reference”). Mirror-imaged designs are formed on opposing inner facing surfaces of two tin-containing polyimide films using conventional micromachining techniques (e.g., chemical or laser etching). When two inner imaged films are subsequently superimposed in mirror relationship, channels and other structures for containing fluid flows may be formed along the interfaces between layers. The tin-containing polyimide layers may be attached by heat-pressure bonding (lamination) carried out at a temperature between 418-441° C. and a pressure between 250-450 psi for a period between 5 and 15 minutes.
Both the Moles and Gumm references require the addition of metallic (i.e., tin) ions to polyimide layers in order to achieve satisfactory bonding. The presence of metal ions limits the utility of such devices in several applications. In one desirable microfluidic application, chromatography, the presence of metal ions would be detrimental for several reasons. For example, metal ions can contaminate stationary phase material, thus rendering the stationary phase material incapable of performing its intended separation of a sample. Additionally, many mobile phase solvents will cause leaching of metal ions into the mobile phase, thus causing detection problems such as extraneous peaks and/or signal drift.
The presence of metal ions can also problematic in other microfluidic applications involving biological moieties. Specifically, metal ions are known to interact with biological materials such as enzymes, proteins, and cells. In microfluidic devices fabricated with metal ions, it may be difficult to execute controlled experiments using biological materials.
Moreover, the Moles and Gumm references are limited to the use of polyimides, which may have limited applicability in certain microfluidic applications. From a material compatibility perspective, polyimides are susceptible to hydrolysis when subjected to alkaline solvents, thus precluding their reliable use in applications such as chemical synthesis. Optical properties present another drawback in using polyimides. Because polyimides are generally opaque to many useful light spectra, they are ill-suited for use with many proven detection technologies that are commonly used in analytical chemistry. Further, an impaired ability to see into microfluidic devices also inhibits experimental use and quality control verification. Finally, both the Moles and Gumm references disclose the fabrication of channels using time-consuming surface micromachining techniques such as photolithography coupled with etching techniques, which requires high setup costs.
Adhesiveless bonding of polymer layers is generally well-known and widely used for applications outside the regime of microfluidic device fabrication. For example, a textbook describing polymer bonding is “Joining of Plastics: Handbook for Designers and Engineers” by Jordan Rotheiser, Hanser Gardner Publications, Inc., Cincinnati, Ohio (1999). Typically, if a structure having more than two layers is desired, such a structure is fabricated one interface at a time. In other words, most conventional multi-layer polymer structures are fabricated sequentially, such as by first joining two thin polymer films to yield a thick film, then joining another thin film to the thick film to yield a thicker film, and so on. Conventional methods include hot plate welding and hot gas welding (wherein which the surface(s) of one or both layers to be joined are melted and then contacted or pressed together) and hot roll lamination involving one or more heated rollers. Conventional applications for laminated polymer layers do not require precise inter-layer alignment—but even if such alignment were required, sequential joining methods would be limiting in this context because each joining step inherently causes significant dimensional distortion (e.g. thinning, shrinkage in one direction, and/or elongation in another direction), and the combined effect of several distortions would render precise alignment practically impossible.
Moreover, in typical applications involving adhesiveless bonding of polymer layers, small internal features are not present, and the practitioner need not be concerned with maintaining the integrity of such features. As a result, in most applications involving adhesiveless polymer layer bonding, the polymer interfaces are melted to a degree that there is significant material flow at the interface to achieve maximum bond strength. Thus, if precise features were provided in the layers prior to bonding, such features would be unlikely to survive conventional bonding processes.
In light of the foregoing, it would be desirable to be able to fabricate adhesiveless and substantially metal-free microfluidic structures having high bond strength (so as to permit leak-free fluidic operation at high pressures), yet being free of collapsed regions (that would lead to unpredictable fluid flow within the microstructures). It would be desirable if such structures could be fabricated from a range of different materials, including substantially colorless materials to provide compatibility with established optical detection techniques and quality control methods. It would also be desirable if such structures could be fabricated with minimal dimensional distortion of internal and external features. It would be further desirable if such structures could be prototyped and fabricated quickly at a low cost.